Planning and preservation officials gathered Tuesday on the observation tower of City Hall, a historic landmark in its own right, to roll out their new website, HistoricPlacesLA.org.

Fifteen years in the making, it is the joint project of The Getty Conservation Institute and the city of Los Angeles.

So far, it has resulted in the survey of about three-quarters of LA's 880,000 structures and placed about 25,000 determined to be the most significant onto the website's database. When the survey is completed next year, every home, office building, freeway, bridge and iconic structure like the Hollywood sign will have been reviewed.

Officials say the undertaking will not only help the city keep track of its historic resources, but also help tourists find them and possibly prevent developers from tearing any more of them down.

"A city of the future has to be built on the past, and it has to be built on a past that is preserved," said James Cuno, president and CEO of the J. Paul Getty Trust, which oversees The Getty Conservation Institute, which began the study in 2000.

The resulting website and database, which went live Monday night, can be accessed by anybody with a smartphone. That will be a great guide to someone considering replacing an old building with a new one, developer Wayne Ratkovich said. "Before Survey LA, developers and property owners were largely in the dark," he said.

Included in the database are architectural masterpieces such as the Griffith Park Observatory and the LA homes and public buildings designed by Paul R. Williams, the first black member of the American Institute of Architects and known in the 1920s and 1930s as "the architect to the stars."

LA's quirky side is also represented with structures like Chamberlain's home and Los Angeles International Airport's space-age looking "Theme Building," with its now-closed rotating restaurant.

"There's still a persistent and unfortunate myth about Los Angeles that we hear all too often, that we're a city that has little or no significant architecture and that just doesn't' care about its past," said Ken Bernstein, director of LA's Office of Historic Resources. "The rich findings in HistoricPlacesLA certainly explode that myth once and for all."